Development vaults semiconductor technology eight years into the future

New high-resolution lithographic techniques developed by researchers at the College of Engineering's Center for NanoTechnology and UW-Madison Synchrotron Radiation Center could allow chipmakers to write semiconductor features with dimensions as small as 20 nanometers.

Smaller features means more transistors can be fit on a chip. More transistors on a chip mean faster microprocessors.

According to Moore's Law, the number of transistors on a square inch of integrated circuit will double every 12 to 18 months. With the "bright peak, enhanced X-ray phase-shifting mask (BPEXPM)", invented by Chemistry Professor James Taylor, researcher Lei Yang and Electrical and Computer Engineering Professor Franco Cerrina, semiconductor technology takes a leap into the future.

"With this bright peak technology, you could write a 100 nanometer mask feature and wind up with a 20 nanometer wafer feature," says James Taylor, John Bascom Professor of Chemistry, Emeritus. "That is about 2010 on the semiconductor industry roadmap. It's really exciting. The masks are not that difficult to create. We have undergraduates that can create a 250 nm mask without too much trouble. Using bright peak masks they would get 50 nanometer wafer features."

Strong coupling of two peaks from two edges to produce high intensity bright narrow peak. (large image)

Mask positioning allows for the creation of the bright peak. (large image)

Features are imaged onto silicon wafers via lithography. To make a modern microprocessor like the Intel Pentium 4, chipmakers use lenses to focus an ultraviolet-light wavelengths of 248 or 193 nanometers on a mask containing one level of the circuit pattern. The mask is projected over a silicon wafer covered with light sensitive material known as photoresist. Light shining through the mask produces pattern features as small as 100 to 110 nanometers in the photoresist. The unexposed portion is washed from the chip.

But to push microchip fabrication below 100 nanometers, researchers need new technical solutions. For example, working with shorter wavelengths allows for smaller chip features, but quartz lenses intended to focus and reduce the circuit pattern onto the silicon absorb much of the light. So the next generation of lithographic techniques must work with special lens materials or without lenses. Center for NanoTechnology scientists have developed new techniques to pattern on the scale of 50 nm and smaller using X-rays. It was in exploring the limits of X-ray lithography that researchers developed the bright peak technology.

"There are several factors that play into the fabrication of smaller features, one of which is diffraction," says McFarland-Bascom Professor and NanoTechnology Center Director Franco Cerrina. "We can control diffraction using phase-shifting techniques and turn it to our advantage. This works well in optical lithography, and even better in X-rays." BPEXPM technology gets beyond one-dimensional resolution and leapfrogs development on the semiconductor industry roadmap by positioning clear, adjacent phase-shifting features to take advantage of constructive interference that occurs at the edges. The interference is bent toward the center of the structure where it combines to form a bright peak. The sharpness of the peak determines the final site of the image. The result is a wafer feature five to six times smaller than the opening written in the mask.

"The maximum intensity and the sharpness of the peak depends on the thickness of the mask material or phase angle, the wavelength of the light used, and the width of the opening," says Taylor. "We are doing research now to determine how to best make the mask, which phase shifter materials to use, what resist materials produce the best resolution and sensitivity, and the long-term stability of the membrane and the phase shifter."

The Wisconsin Alumni Research Foundation (WARF) holds the patent rights on the technology, and a patent was issued on August 6, 2002 as US 6,428,939. WARF is prepared to license the technology to interested companies.